1. Field of the Invention
The present invention relates to a PLL (Phase-Locked Loop) synthesizer and, more particularly, to a PLL synthesizer capable of outputting signals lying in two or more different frequency bands.
2. Description of the Background Art
It is a common practice with a portable telephone to effect communication by using two different frequency bands, i.e., a 800 MHz to 900 MHz band and a 1.5 GHz or 1.9 GHz band. In the market, however, there is an increasing demand for a portable telephone capable of covering two different frequency bands, e.g., 800 MHz band and 1.5 GHz band alone. To meet this demand, a PLL synthesizer included in a portable telephone either for local oscillation or for modulation is required to cover the two different frequency bands.
However, because the above two frequency bands are remote from each other, it is difficult for the PLL synthesizer to cover both of the two frequency bands with a single VCO (Voltage Controlled Oscillator) from the stable frequency security standpoint. In light of this, the PLL synthesizer may be provided with two VCOs each oscillating a signal lying in a particular frequency band, and a switch for selecting either one of the signals output from the VCOs. However, the provision of the PLL with the two VCOs and the switch scales up the circuitry of the PLL synthesizer to thereby increase the overall size and cost of a portable telephone set including the synthesizer.